This invention relates to a method for fast adhesion of a silver coating to nitride type ceramics.
Nitride type ceramics, excelling particularly in strength, resistance to thermal shocks and resistance to chemicals, have come to gain enthusiastic acceptance as refractory materials for use in applications the conventional oxidative type ceramics have failed to satisfy. In consequence of the recent development of new fields for the applications of high-temperature techniques, industries concerned are showing an inclination toward utilizing nitride type ceramics in special high-temperature insulators as well as in machine parts such as those in waste gas purification systems which are destined to be elevated instantaneously to high temperatures.
In many if not all cases, ceramic articles intended for industrial applications have their surface metallized, i.e. covered with thin coatings of various metals, in advance to their actual use.
Methods for the metallization suggested to date to the art include the molybdenum method and the silver-coating fusion method which is disclosed by Japanese Pat. No. 169,251. These methods invariably require the thermal treatment to be performed under vacuum or in an atmosphere of hydrogen gas and, therefore, call for highly advanced techniques. With a view to improving the methods described above, one of the present inventors has suggested a method for fast adhesion of a silver coating to the surface of an alumina ceramic article, which comprises first coating the surface of the alumina ceramic article with a mixture of copper sulfide and kaolin, heating the coated article in the atmosphere of an oxidative gas at a temperature of not less than 900.degree. C. for thereby inducing a reaction of the reactants present, subsequently overcoating the thermally treated ceramic article with silver carbonate and heating it at a temperature of not less than 700.degree. C. (Japanese Patent Publication No. 21569/1972).
These methods which have been suggested to date, when applied to adhesion of metals to nitride type ceramics, have invariably failed to provide fast adhesion of metals because the nitride type ceramics exhibit a very poor wetting property to molten metals.
An object of the present invention is to provide a method capable of fast, powerful adhesion of a silver coating to the suface of nitride type ceramics which have so far permitted no easy adhesion of metals by fusion.